b
\(Image\)

\(F_{P S} \propto Q^2\)

\(F_{P S}=16 N\)

Now If \(\mathrm{P} \& \mathrm{R}\) are brought in contact then

\(Image\)

Now If \(\mathrm{S} \& \mathrm{R}\) are brought in contact then

\(Image\)

New force between \(P \& S\) is :

\(\mathrm{F}_{\mathrm{PS}} \propto \frac{\mathrm{Q}}{2} \times \frac{3 \mathrm{Q}}{4}\)

\(\mathrm{~F}_{\mathrm{PS}} \propto \frac{3 \mathrm{Q}^2}{8}=\frac{3}{8} \times 16=6\)